It has become increasingly important in a number of various and diverse industries to have a source of high purity process gas. In the electronics industry, for example, reactive gases such as silane, arsine, diborane, phosphine, sulfur hexafluoride, hydrogen bromide, tungsten hexafluoride, and others are widely used in the semiconductor industry in the manufacture of integrated circuitry. The etching and deposition processes for new semiconductor designs often require extremely low levels of measurable contamination. Transfer lines have to be essentially free of contamination. Use of reactive gases that may be pyrophoric and extremely toxic present significant safety issues when changing the gas supply.
Delivery of the process gases to a point of use is made through gas transfer conduits comprised of valves and connections. These conduits are commonly connected to a source such as gas cylinder, tube trailer, etc. and to a point of use delivery site, e.g., a gas control manifold. Purging the gas transfer conduit of piping, tubing and valves, commonly called a pigtail, is a common practice. On the one hand, purging is effected before disconnection of the gas source to eliminate any gas remaining in the conduit that may be hazardous to the operator. And, after connection of the pigtail to the gas source, e.g., a gas cylinder, purging is effected for the purpose of removing contaminants, e.g., moisture and atmospheric gases now present in the pigtail. In the step of purging, the pigtail is pressurized and de-pressurized multiple times during the change-out of an empty high-purity gas cylinder, and its replacement.
As in change-out of gas sources for the electronic industry, other industries face similar issues. For example, purging of pigtail conduits employed in analytical apparatus is performed in those instances where one wishes to change out the carrier gases associated with the analytical process. Purging can be done in similar manner to that in the electronics industry.
At present, there are several basic techniques used in the industry to purge the pigtail connection such as that employed between a delivery site manifold and a replacement high purity gas cylinder. The most widely used dilution techniques for purging are known as “cross-purge” and “deep-purge”.
Cross Purge Dilution purging is a succession of manifold pressurizations and depressurizations. The sources providing the pressurization and depressurization are mounted away from the connection to the gas source. Typically, the manifold and pigtail flow lines containing the process gas are vented to a low-pressure system maintained at or below atmospheric pressure. The control manifold is thereafter pressurized, typically to several atmospheres, with a purge gas, typically an inert gas such as nitrogen, and again vented to the low-pressure or vacuum system. Each successive application of venting and pressurizing constitutes a purge cycle. The procedure is repeated for a predetermined number of cycles over a period of time until the process gas concentration reaches a low level considered safe and clean for the semiconductor fabrication process.
Deep-purge has been widely practiced by introducing a purge gas at or near the cylinder valve connection. For example, a system may be provided for supplying purge gas to within about 2 inches of the cylinder valve outlet. Deep purge improves contaminant removal by eliminating the “dead volume” in the “pigtail” and particularly in the cylinder connection itself.
A current method of implementing the “cross-purge” and “deep-purge” techniques includes the use of a vacuum generator to accelerate the evacuation of the manifold.
Representative patents illustrating delivery processes of high purity gases employing impurity removal methods are as follows:
U.S. Pat. No. 5,137,047 discloses a system for delivery of a reactive gas for semiconductor applications comprising a pigtail for connecting the supply source with the point of use and a purge gas subsystem. The pigtail has an orifice said to allow the required flow of process gas while eliminating reverse diffusion of atmospheric gases. The purge gas line is connected to the pigtail so that when a vacuum is produced in the delivery circuit, a purge gas can be admitted to remove moisture and impurities. A venturi is used to create a vacuum during the purge cycle.
U.S. Pat. No. 5,359,787 discloses chemical transfer apparatus for delivering corrosive chemicals from a tube trailer while reducing moisture and entrainment of particulates. A purge system employing a vacuum source is used. Argon is used as a purge gas in order to avoid entrainment of atmospheric air and then it is vented.
U.S. Pat. No. 5,749,389 discloses apparatus for delivering high purity gas from gas cylinders to a point of use in semiconductor operations. An improved system to the prior art cross flow, deep purge and vacuum generator methods for purging is shown and the improved system comprises: a process connection through which ultra-high purity gas is in communication to a process; a first pigtail conduit in selective flow communication with a high purity gas source; a second pigtail conduit in selective flow communication with said process connection; a source of vacuum and a connection for selectively placing said vacuum source in communication with said first pigtail; and an ultra-high purity process gas source. The preferred embodiment further comprises a block valve assembly comprising an inlet port in flow communication with said high-purity process gas source, a first outlet port in selective flow communication with said purge gas supply through connection with said first pigtail, and a second outlet port in selective flow communication with said process connection through connection with said second pigtail. A pigtail bleed is used to prevent atmospheric air from causing contamination.
U.S. Pat. No. 5,398,712 discloses a device for use in the removal of contaminants from a gas cylinder valve assembly using vacuum and purge techniques. The device uses a purge gas inlet having a first orifice cross-section, a purge gas outlet having a second orifice cross-section and a third orifice connection for the first and second orifices having reduced cross-section to the first ad second orifice. A fourth orifice is connected to the gas cylinder and is joined to the second orifice. As purge gas is passed through the first orifice to the outlet, a vacuum is created in the fourth orifice connected to the gas cylinder and such vacuum facilitates removal of contaminant gas.